Quickest way from Black Forest to Munich to take a side trip to the Neuschwanstein Castle

The fastest way by train is 6 hours to Fuessen, and you should get going early. Current schedule says leave at 6:40 am to give you your 3 to 4 hours at the castle. September is a long way off, so check the schedule when you get closer.

If you mean taking a side trip to see the castles once you are in Munich, then there are frequent trains to Fussen that take about 2 hours each way, with one change of trains. The bus from Fussen to the ticket centre (about every half hour) takes less than 10 minutes. Then you need to get from the ticket center to the castle - at least another half hour. Reverse for the return trip. The only advance reservation needed is your timed entry for the castle (or castles if you also visit Hohenschwangau, which I highly recommend). The trains are regional so you can just buy your tickets at the station before you leave. You could buy one Bayern ticket if your group is 5 or less. Hop on the next train and take any seat. Buy your bus ticket (cash only, I believe) directly from the driver.

The wording of your question leads me to believe you have not thoroughly researched this topic. It's difficult to give specific answers to vague questions.

Are you asking how long it will take from Triberg to Munich? Are you then taking a day trip to see Neuschwanstein?

Assuming I'm going to walk around the Castle for 3 or 4 hours

Do you mean you are going to walk around the outside of the castle for 3 or 4 hours? You can't walk around in the inside of the castle for 3 or 4 hours. You have to take a guided tour through the castle; it's a fixed time (approx. 30 min).

My wife and I will be traveling in September from the Triberg Black
Forest area to Munich on the db train. What is the quickest way to
take a side trip (and from which town) to the Neuschwanstein Castle.

A "side trip" to N'stein might be understood as either...

1) ...a stop that you make on the same day as you travel between Triberg and Munich, or...

2) ...a round-trip journey from Munich to N'stein - probably NOT the same day but on some later day during your Munich stay.

You can find the quickest way for these two options by using the DB site. It will include the final shuttle bus trip between Füssen's train station and the bus stop for N'stein.

https://int.bahn.de/en/

1) From Triberg / To Munich

• Add a stopover period of 3-4 hours using this stop: Neuschwanstein Castles, Schwangau
• The final train terminates at Füssen station, where you can stow bags in lockers prior to boarding the shuttle bus.

2) From Munich / To Neuschwanstein Castles, Schwangau

• (Swap stations to find a schedule for the reverse trip.)

Of course, for 2), you have to get to Munich in the first place on an earlier day; use Triberg and Munich for that trip.

My comments: #1 and #2 are both ridiculously long travel days with too many changes of train, not a trip I would willingly choose to make. Your core problem is an itinerary that connects Triberg with distant destinations in Bavaria. The area between Triberg and Bavaria is not useless wasteland but full of nice destinations. What I might do in your situation...

Leave Triberg in the afternoon for ULM (roughly 3 hours away, on the Bavarian border) for a night there. Visit the Ulm cathedral (world's tallest church steeple) and enjoy a couple hours' walk around the old part of town. Ulm is one of those really nice, normal German cities that gets neglected by guidebooks that emphasize cutesy-cutesy Germany. Travel to either N'stein or Munich the next day.

Thanks for all the replies. I realized the interior of the Neuschwanstein Castle is 30 minute guided tour. I was referring to walking around the grounds as well as the tour when I mentioned 3 to 4 hours. But all this helps!

I may just plan an overnight at Fussen, do the castle's the next day and then overnight night again at Fussen before continuing back to Munich. That would give me an entire day for the two castles and walk around a bit. Thanks All!

Having a whole day in September you might add a walk around the Lake Alpsee near Hohenschwangau Castle.

Its a 2hrs (5km) curcuit walk around the lake with views of Hohenschwangau and Neuschwanstein Castle

https://www.outdooractive.com/de/route/wanderung/allgaeu/alpseerundweg/2808335/#dmlb=1

video

Thanks!!!!